The Montsabrais summit caldera: volcanology, volcano-tectonic ring-dyke structures,
timing, and hydrothermal alteration of a potential VMS-deposit

Projet DIVEX-B SC42


Responsable du projet

Wulf Mueller, UQAC, courriel

Introduction

The innovative study of Pearson and Daigneault (2009) allowed a complete re-interpretation of the 2900 km2 Archean Blake River Group (BRG) in Abitibi greenstone belt, as a submarine megacaldera complex. The study identified two new calderas: 1) 40x80 km Misema (MC) and 2) 14x35 km New Senator calderas (NSC). The graben-type 15x20 km, Noranda caldera had been initially documented by de Rosen-Spence (1976) and most of the principal VMS (volcanic massive sulfide) deposits were associated with the central Mine Sequence. With the identification of these calderas and their geometry as well as structure, a fundamental change in the philosophy of BRG mineral exploration emerged.

The MC is the oldest of the three calderas (ca. 2706-2704 Ma) and is comparable in size to that of Toba (ca. 30-80 km in diameter). The MC is an amalgamation of at least three large mafic shield volcanoes. Based on this megacaldera interpretation, new exploration targets have been chosen, including the Montsabrais (MVC) and Renault volcanic centres (RVC; e.g. Dimroth et al., 1985) in the northern segment of the Misema caldera (MC). The MVC and RVC are located on the margin of the MC between the inner and outer ring caldera faults and are an integral part of a series of 2-12 km in diameter diorite-gabbro ring dyke complexes. Special focus is on the MVC because of the strong VMS potential due to the abundance volcaniclastic-pyroclastic deposits, pillowed and massive flow units, an extensive hydrothermal alteration pattern and favourable VMS showings. This emission centre has been interpreted now as a mafic summit caldera with major volcanic activity (Mueller et al, 2009a, b).

Subaqueous caldera settings are first order loci for VMS deposits as shown by recent models by Stix et al., (2003) and modern VMS deposits at caldera margin walls (Iizasa et al., 1999; Schmincke, 2004; Acocella, 2007). In fact, the largest VMS deposits are located along the annular faults. In contrast, Abitibi exploration companies have paid little attention to the ring dyke complexes. These ellipsoidal collapse structures are common to all shield volcanoes and are primes sites of VMS activity in the modern-day setting. The ring dyke complexes according to Mueller et al. (2009b) are Archean analogues, whereby the dykes and sills are considered the deeper roots and hence plumbing system of the summit calderas. These mafic sills and dykes associated with synvolcanic circular faults represent a sustained heat generator over several million years that can produce an extensive hydrothermal VMS field (e.g. the Ashes, Axial Seamount; Baker et al., 1999).

Summit caldera margin

Location of hydrothermal vents on the summit caldera margin of Axial Seamount. The ca. 3x8 km mafic summit caldera is located along the Juan de Fuca Ridge (after Schmincke, 2004). The MVC has a strong VMS potential.

Objectives

The prime objective of this study is to understand the mechanism an Archean ring dyke complex (summit caldera). Deciphering an Archean subaqueous summit caldera system is novel. The research group will use several approaches that are complimentary. Student and company mapping will give a more complete picture of the MVC concerning:

  1. VMS potential
  2. hydrothermal alteration pattern
  3. synvolcanic faulting
  4. physical volcanology
With the help of precise U-Pb baddelyite geochronology the group intends on defining the timing of such an Archean summit caldera system. All the various dykes and sills related to the summit caldera will be sampled (16 samples collected over two years), because the duration of such a volcanic event is unknown and has yet been documented. Such systems generally 3-10 m.y.

Methodology

The proposed research has two princial components:

  1. A field component with a classical volcanic and sedimentary facies analysis approach. The various volcanic and volcaniclastic (sedimentary) units will be mapped in detail (scales 1:100 to 1:2000) in order to understand the transport or emplacement process with the aid of a rigorous sampling programme for the hydrothermal alteration pattern and a structural analysis of the study area.
  2. An important laboratory component. We will analyse the componentry and geochemistry of lava flows, dykes-sills and subaqueous volcaniclastic deposits. The petrography will be used to identify delicate pyroclasts and alteration minerals. The important U-Pb geochronology will be conducted at UBC.

References

Acocella, V., 2007. Understanding caldera structure and development: an overview of analogue models compared to natural calderas. Earth-Sci. Rev. 85, 125-160.

Baker, E.T., Fox, C.G., Cowen, J.P., 1999. In situ observations of the onset of hydrothermal discharge during the 1998 submarine eruption of Axial Volcano, Juan de Fuca Ridge. Geophysical Research Letters 26, 3445–3448.

de Rosen-Spence, A.P., 1976. Stratigraphy, development and petrogenesis of the central Noranda volcanic pile, Noranda, Quebec. Unpublished PhD thesis, University of Toronto, Canada, 166 pp.

Dimroth, E., Imreh, L., Cousineau, P., Leduc, M., and Sanschagrin, Y., 1985. Paleogeographic analysis of mafic submarine flows and its use in the exploration for massive sulfide deposits. In: Ayres, L.D., Thurston, P.C., Card, K.D., Weber, W. (Eds.), Evolution of Archean Supracrustal Sequences. Geological Association of Canada, Special Paper, vol. 28, pp. 203–222.

Iizasa, K., Fiske, R. S., Ishizuka, O., Yuasa, M., Hashimoto, J., Ishibashi, J., Naka, J., Horii, Y., Fujiwara, Y., Imai, A., and Koyama, S., 1999, A Kuroko-type polymetallic sulfide deposit in a submarine silicic caldera: Science, 283, 975- 977.

Mueller, W.U., Stix, J., Corcoran, P.L and Daigneault, R., 2009a. Subaqueous calderas in the Archean Abitibi greenstone belt: An overview and new ideas. Ore Geol. Rev., 35, 4-46.

Mueller, W.U., Daigneault, R., Lafrance.,B, Stix, J, Corcoran, P.L., and Pearson, V., 2009b. Abitibi subaqueous calderas with volcanogenic massive sulfide deposits: exploration models and hydrothermal carbonate alteration. In Submarine volcanism and mineralization: modern through ancient, B. Cousens and S.J. Piercey (editors), Submarine volcanism and mineralization: modern through ancient. Geological Association of Canada, Mineral deposits Division, Short Course Notes, volume 19, 61-90 (plus figures).

Mueller, W.U., Stix, J., White, J.D.L., Corcoran, P.L., Lafrance, B., and Daigneault, R., 2008. Characterization of Archean subaqueous calderas in Canada: physical volcanology, carbonate-rich hydrothermal alteration and a new exploration model. In: J. Gottsmann and J. Martí (Editors), Caldera volcanoes: analysis, modeling and response. Developments in Volcanology 10, Chapter 5 Elsevier, pp 183-232.

Pearson, V. and Daigneault, R., 2009. An Archean megacaldera complex: the Blake River Group, Abitibi greenstone belt. Precamb. Res.,168, 66–82.

Schmincke, H.-U., 2004. Volcanism. Springer Verlag, Heidelberg: 324pp.